The human SIL gene is disrupted by a site-specific interstitial deletion in 25% of children with T-cell acute lymphoblastic leukemia. Since transcriptionally active genes are prone to recombination events, the recurrent nature of this lesion suggests that the SIL gene product is transcriptionally active in the cell type that undergoes this interstitial deletion and that the SIL gene product may play a role in normal lymphoid development. To facilitate studies of SIL gene function, we have cloned and characterized a murine SIL gene. The predicted murine SIL protein is 75% identical to the human gene, with good homology throughout the open reading frame. An in vitro translated SIL cDNA generated a protein slightly larger than the predicted 139-kDa protein. Although a prior report detected SIL mRNA expression exclusively in hematopoietic tissues, a sensitive RT-PCR assay demonstrated SIL expression to be ubiquitous, detectable in all tissues examined. Since the RT-PCR assay suggested that SIL mRNA expression was higher in rapidly proliferating tissues, we assayed SIL mRNA expression using a murine erythroleukemia model of terminal differentiation and found it to be dramatically decreased in conjunction with terminal differentiation. These studies demonstrate that the human SIL gene product is quite well conserved in rodents and suggest that the SIL gene product may play a role in cell proliferation.